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A novel silicon-oxygen aurone derivative assisted by graphene oxide as fluorescence chemosensor for fluoride anions
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 182 (2017) 37–41
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A novel silicon-oxygen aurone derivative assisted by graphene oxide as fluorescence chemosensor for fluoride anions Yongxiao Xu a, Qinghua Yang b, Duxia Cao a,⁎, Zhiqiang Liu c, Songfang Zhao a, Ruifang Guan a,⁎, Yibing Wang a, Qianqian Wu a, Xueying Yu a a b c
School of Material Science and Engineering, University of Jinan, Jinan 250022, Shandong, China School of Material Science and Engineering, Shandong University, Jinan 250061, Shandong, China State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, Shandong, China
a r t i c l e
i n f o
Article history: Received 12 December 2016 Received in revised form 24 March 2017 Accepted 31 March 2017 Available online 2 April 2017 Keywords: Aurone derivative Graphene oxide Chemosensor Fluoride anion
a b s t r a c t A novel silicon-oxygen aurone derivative TBDPSA was synthesized and used for the detection of fluoride anions in aqueous solution based on a specifically F−-triggered silicon-oxygen cleavage. Even though the compound has shown high selectivity, obvious absorption and fluorescence response for fluoride anions in aqueous solution, but it also is suffered from many limits, such as low detection sensitivity and long response time. Here the compound was successfully assembled on the graphene oxide (GO) surface by π-π stacking. GO improves recognition sensitivity and shortens response time of TBDPSA for fluoride anions by taking advantage of the nanocarrier GO. Compared with TBDPSA, the response time of GO/TBDPSA is shortened greatly from 1 h to b 5 s and the detection limit is lowered about four times with fluorescence as detected signal. Generally speaking, GO is an excellent promoter for accelerate recognition. © 2017 Elsevier B.V. All rights reserved.
1. Introduction In the last few years, the chemosensors to identify and detect biological and environmental important species have developed rapidly and become a hot topic [1–5]. At the same time, many scientists are working on the innovation in some systems to overcome the shortcomings of the previous ones [6–8]. The research about the recognition for fluoride anions, which can prevent tooth decay and clinical osteoporosis, is an interesting example. As a kind of anions, which has minimum ionic radius, highest electron cloud density, hydrolysis ability and a Lewis basic nature, the study for fluorine anion recognition has attracted many researchers' interest [9–13]. In current research, fluoride anion chemosensors can be divided into several main types: hydrogen bonding receptors [14,15], organoboron compounds [16–19], metal complexes [20,21] and F− promoted cleavage of silicon-oxygen bonds [22–26]. The first two types of chemosensors normally cannot be used in aqueous solution. Since Si\\F bond has more dissociation energy than Si\\O bond (ESi\\F = 141 kcal/mol, ESi\\O = 103 kcal/mol) [9], Si\\O bond is easier to be combined with fluoride anion. Due to this irreversible chemical process, F− chemosensors can be developed to realize obvious recognition in aqueous solution with high selectivity. But it has its inherent imperfection. Since this reaction type of chemosensors ⁎ Corresponding authors. E-mail addresses: [email protected] (D. Cao), [email protected] (R. Guan).
http://dx.doi.org/10.1016/j.saa.2017.03.073 1386-1425/© 2017 Elsevier B.V. All rights reserved.
have long reaction time and molecular reaction potential barrier, the identification process needs a certain number of fluoride anions to break Si\\O bond. Graphene oxide (GO) possesses some unique excellent characteristrics [27,28], such as large surface area, high water dispersibility and catalytic action, which can absorb chemosensor molecules on its surface through π-π stacking and improves the hydrophilia of chemsensor as nanocarrier [29–32]. Our group has devoted to fluoride recognition for about ten years [33–35]. Here, a novel silicon-oxygen aurone derivative TBDPSA was synthesized and successfully assembled on the GO surface. As expected, the nanocomposite GO/ TBDPSA overcomes the disadvantage of long response time and low sensitivity of TBDPSA.
2. Experimental 2.1. Chemical and Instruments The synthetic routes to the title compound TBDPSA is shown in Scheme 1. 4′-Bromide-4-hydroxy-aurone (S-1) was synthesized according to ref. [35]. tert-Butyldiphenylsilyl chloride was obtained from Aladdin Reagent Inc. Nuclear magnetic resonance spectra were obtained on a Bruker Avance III 400 MHz spectrometer. Mass spectra were recorded on an Agilent Q-TOF6510 spectrometer. Scanning electron
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Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 182 (2017) 37–41
ethanol solution with GO in HEPES buffer solution. Fluorescence spectra were obtained with the excitation wavelength being 389 nm. Both the concentration of pure TBDPSA and composite GO/TBDPSA used for spectral measurement is 10 μM. KF in HEPES buffer solution was used as fluoride anions source. Scheme 1. Synthetic routes to the title compound TBDPSA.
microscope (SEM) and energy-dispersive X-ray (EDS) analysis was recorded by FEI QUANTA FEG 250 scanning electron microscope.
2.5. Structure Determination Single crystal of compound TBDPSA was obtained by slow evaporation of the compound in dichloromethane-n-hexane mixed solution. The diffraction data were collected on a Bruker Smart Apex CCD area detector using a graphite mono-chromated Mo Kα radiation. Crystal data, data collections and structure refinements of TBDPSA are shown in Table S1.
2.2. The Synthesis of TBDPSA 3. Results and Discussion S-1 (0.10 g, 0.32 mmol), imidazole (0.027 g, 0.4 mmol) and 3 mL dichloromethane were poured in a 25 mL flask. After cooling to 0 °C, 0.11 mL (0.09 g, 0.33 mmol) tert-butyldiphenylsilyl chloride was dropped into the solution. And then the mixture was stirred at room temperature for 2 h. The crude product was evaporated under reduced pressure and then oil sample was obtained. The product was purified via column chromatography with ethyl acetate/petroleum ether (1:4) as eluent to give 0.11 g TBDPSA as a yellow solid (62% yield). 1H NMR (400 MHz, CDCl3), δ: 1.18 (s, 9H), 6.13 (d, J = 8.0 Hz, 1H), 6.76 (s, 1H), 6.78 (d, J = 8.0 Hz, 1H), 7.17 (t, J = 8.0 Hz, 1H), 7.38–747 (m, 6H), 7.57 (d, J = 8.4 Hz, 2H), 7.76 (d, J = 8.4 Hz, 2H), 7.77–7.79 (m, 4H). 13C NMR (100 MHz, CDCl3), δ: 19.88, 26.48, 104.96, 110.07, 113.13, 114.37, 123.94, 128.08, 130.35, 131.68, 132.20, 132.23, 132.69, 135.66, 137.78, 147.49, 154.87, 166.86, 182.50. MS for (M + Na)+: Calcd exact mass 577.0811; found 577.0741. 2.3. The Synthesis of Nanomaterial GO/TBDPSA GO was prepared with natural graphite as starting material using modified Hummer's method [36]. TBDPSA was added to the diluted stock solution of GO (0.5 mg/mL). After the mixture was stirred for 2 h, GO/TBDPSA and the dissociative TBDPSA were separated by dialysis way. Finally, the GO/TBDPSA product was dried under vacuum at 60 °C. The chemical composition and surface structure of GO and GO/TBDPSA were characterized by SEM and EDS spectroscopy. 2.4. Photophysical Properties' Measurements UV–vis absorption and fluorescence spectra with C = 10 μM at room temperature were obtained on a Shimadzu UV2550 spectrophotometer and Edinburgh FLS920 spectrometer, respectively. GO/TBDPSA solution used for spectral measurement was prepared by mixing TBDPSA in
3.1. Synthesis and Molecular Structure The compound was synthesized by the reaction between tertbutyldiphenylsilyl chloride and hydroxy in 4′-substituted 4-hydroxyaurone. TBDPSA was purified and fully characterized via 1H NMR, 13C NMR, mass spectrum and crystal structure. The molecular structure of TBDPSA is shown in Fig. 1. The molecular belongs to triclinic system and P1 space group. The molecular backbone exhibits perfect planarity with the dihedral angle between benzofuran and benzene group being 8.87°, which is similar with that of the hydroxyl aurone precursor [35]. 3.2. Response of the Compound to Fluoride Anions The compound TBDPSA was dissolved in aq. HEPES buffer-ethanol solution (1:1, v/v) and fluoride anions was progressively added in the solution. UV–vis absorption and fluorescence titration spectra are shown in Fig. 2. As shown in the Fig. 2a, the compound exhibits strong UV–vis absorption in blue light region with absorption peak at 305 nm (ε = 4.68 × 104 M−1 cm−1) and 389 nm (ε = 4.24 × 104 M−1 cm−1) in aq. HEPES buffer-ethanol solution (1:1, v/v). Upon the addition of F−, the original absorption peak at 389 nm gradually decreases with a little bathochromic shift. TBDPSA solution would reach saturation when 1000 equiv. F− was added. The fluorescence spectra of the compound also changes obviously during the titration with F−, which was shown in Fig. 2b. The fluorescence peak at 498 nm gradually decreases and is quenched finally. During the titration with KF, the change can be seen by naked eye under UV lamp. Green fluorescence is almost quenched. The bonding constant is calculated to be 3.9 × 104 M−1 [37]. The detect limits of the compound are 76 μM and 0.22 μM with absorption and fluorescence as detected signal, respectively. U.S. Environmental Protection Agency limit the fluoride anions in drinking water are
Fig. 1. Molecular structure of the title compound TBDPSA.
Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 182 (2017) 37–41
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didn't lead to obvious change in the absorption and fluorescence spectra of the compound. Other anions also didn't interfere with the recognition reaction between TBDPSA and F−. 3.3. The Composite GO/TBDPSA Response to Fluoride Anions GO and GO/TBDPSA were first investigated by SEM and EDS spectroscopy (Fig. S3 and S4). As shown in Fig. S3, GO and GO/TBDPSA exhibit good film structure. Comparing with the different EDS peaks between GO and GO/TBDPSA (Fig. S4), there are two more peaks represent Si and Br elements, which just come from chemosensor TBDPSA. As shown in Fig. S5, GO dose not exhibit obvious UV–vis absorption band from 300 nm to 600 nm. GO/TBDPSA exhibits two absorption bands at 309 nm and 389 nm, which is similar to that of TBDPSA. EDS and UV– vis absorption data prove the successful assembly of the GO/TBDPSA composite. TBDPSA exhibits low sensitivity and slow response rate for F− (about 1 h) with rate constant being 1.15 × 10−3 s−1 (Fig. S1). Based on superior catalytic and carriers function from Graphene Oxide (GO), GO was imported to TBDPSA aqueous solution to assistant recognizing F−. As shown in Fig. 4a, GO/TBDPSA solution is at the same concentration with previous titration (C = 10 μM) and only 220 equiv. F− can make the reaction reach saturation. The bonding constant is calculated to 1.2 × 105 M, which is three times that of TBDPSA. The import of GO immensely increases the recognition sensitivity of TBDPSA for fluoride anions. The fluorescence spectra of GO/TBDPSA (Fig. 4b) show the
Fig. 2. UV–vis absorption (a) and fluorescence (b, λex = 389 nm) spectral changes of TBDPSA upon titration with KF in aq.-HEPES buffer-ethanol (VHEPES:VEtOH = 1:1) with equilibrium time being 1 h. Insert: absorbance at 389 nm (a) and fluorescence intensity at 498 nm (b) as a function of concentration of KF. Photographs showing the fluorescence change upon the addition of KF in 365 nm UV-light.
0.7–1.2 ppm (37.1–63.6 μM) [38]. The detect limit [39] with absorption as detected signal does not meet the requirement. Although the detect limit with fluorescence as detected signal reaches the requirement from U.S. Environmental Protection Agency, but the response time is too long (about 1 h, Fig. S1). The selectivity and competitive experiments of TBDPSA for F− and other anions were investigated, which is shown in Fig. 3 and Fig. S2. Different from fluoride anions, other anions
Fig. 3. Fluorescence spectral changes of TBDPSA in aq.-HEPES buffer-ethanol (VHEPES:VEtOH = 1:1) upon the addition of various anions. Insert: Changes of fluorescence peak at 498 nm in the presence of various anions in response to F−.
Fig. 4. UV–vis absorption (a) and fluorescence (b, λex = 389 nm) spectral changes of GO/ TBDPSA upon titration with KF in aq.-HEPES buffer-ethanol (VHEPES:VEtOH = 1:1). Insert: absorbance at 389 nm (a) and fluorescence intensity at 492 nm (b) as a function of concentration of KF. Photographs showing the fluorescence change upon the addition of KF in 365 nm UV-light.
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Fig. 5. The reaction mechanism of GO/TBDPSA to detect F−.
same catalysis in the same situation. Response rate increases smartly and the response time is b5 s. The detect limits of GO/TBDPSA are 42 μM and 0.058 μM with absorption and fluorescence as detected signal, respectively, which satisfies the requirement from U.S. Environmental Protection Agency. So GO can improve the recognition speed for fluoride anions and the sensitivity. GO/TBDPSA composite also exhibits perfect selectivity for F− (Fig. S6).
and Universities Science and Technology Foundation of Shandong Province (J16LA08) and the Fund of Graduate Innovation Foundation of University of Jinan, GIFUJN (S1504). Appendix A. Supplementary Data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2017.03.073.
3.4. Reaction Mechanism References In situ 1H NMR spectra of TBDPSA before and after the addition of F− in CD3OD (Fig. S7) were measured to investigate the recognition mechanism. From Fig. S7, one can see that hydrogen proton signals of the compound at 6.78 ppm (Cα, s) and 6.90 (C5, d, J = 8.0 Hz) are upshifted to 6.50 ppm (Cα, s) and 6.26 (C5, d, J = 8.0 Hz), respectively. Other hydrogen proton signals do not exhibit obvious change in 1H NMR spectra. The possible recognition mechanism based on the in situ 1 H NMR spectra data is shown in Fig. 5. Recognition mechanism relies on the different binding energy between Si\\O bond and Si\\F bond. Si tended to separate with O and combined with F during the titration. As shown in the Fig. 5, GO/TBDPSA would occur permutation reaction and fluoride anions would replace aurone part and combine with TBDPSA. So that Si\\O bond was separated because of higher bond energy of Si\\F. Finally, the whole structure changed and caused red shift and fluorescence was quenched. 4. Conclusion In conclusion, a novel silicon-oxygen aurone derivative TBDPSA was synthesized and successfully assembled on the GO to form GO/TBDPSA composite material. Graphene oxide modified composite GO/TBDPSA exhibits higher sensitivity and fast response rate for fluoride anions compared with TBDPSA in aqueous solution because of the catalytic effect from GO. Generally speaking, GO is an excellent promoter for accelerate recognition. Acknowledgements This work was supported by the National Natural Science Foundation of China (21672130, 51373069, 21601065) and the Natural Science Foundation of Shandong Province (ZR2015BL011), Key Research and Development Plan of Shandong Province (2016GSF117004), Colleges
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A novel silicon-oxygen aurone derivative assisted by graphene oxide as fluorescence chemosensor for fluoride anions Yongxiao Xu a, Qinghua Yang b, Duxia Cao a,⁎, Zhiqiang Liu c, Songfang Zhao a, Ruifang Guan a,⁎, Yibing Wang a, Qianqian Wu a, Xueying Yu a a b c
School of Material Science and Engineering, University of Jinan, Jinan 250022, Shandong, China School of Material Science and Engineering, Shandong University, Jinan 250061, Shandong, China State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, Shandong, China
a r t i c l e
i n f o
Article history: Received 12 December 2016 Received in revised form 24 March 2017 Accepted 31 March 2017 Available online 2 April 2017 Keywords: Aurone derivative Graphene oxide Chemosensor Fluoride anion
a b s t r a c t A novel silicon-oxygen aurone derivative TBDPSA was synthesized and used for the detection of fluoride anions in aqueous solution based on a specifically F−-triggered silicon-oxygen cleavage. Even though the compound has shown high selectivity, obvious absorption and fluorescence response for fluoride anions in aqueous solution, but it also is suffered from many limits, such as low detection sensitivity and long response time. Here the compound was successfully assembled on the graphene oxide (GO) surface by π-π stacking. GO improves recognition sensitivity and shortens response time of TBDPSA for fluoride anions by taking advantage of the nanocarrier GO. Compared with TBDPSA, the response time of GO/TBDPSA is shortened greatly from 1 h to b 5 s and the detection limit is lowered about four times with fluorescence as detected signal. Generally speaking, GO is an excellent promoter for accelerate recognition. © 2017 Elsevier B.V. All rights reserved.
1. Introduction In the last few years, the chemosensors to identify and detect biological and environmental important species have developed rapidly and become a hot topic [1–5]. At the same time, many scientists are working on the innovation in some systems to overcome the shortcomings of the previous ones [6–8]. The research about the recognition for fluoride anions, which can prevent tooth decay and clinical osteoporosis, is an interesting example. As a kind of anions, which has minimum ionic radius, highest electron cloud density, hydrolysis ability and a Lewis basic nature, the study for fluorine anion recognition has attracted many researchers' interest [9–13]. In current research, fluoride anion chemosensors can be divided into several main types: hydrogen bonding receptors [14,15], organoboron compounds [16–19], metal complexes [20,21] and F− promoted cleavage of silicon-oxygen bonds [22–26]. The first two types of chemosensors normally cannot be used in aqueous solution. Since Si\\F bond has more dissociation energy than Si\\O bond (ESi\\F = 141 kcal/mol, ESi\\O = 103 kcal/mol) [9], Si\\O bond is easier to be combined with fluoride anion. Due to this irreversible chemical process, F− chemosensors can be developed to realize obvious recognition in aqueous solution with high selectivity. But it has its inherent imperfection. Since this reaction type of chemosensors ⁎ Corresponding authors. E-mail addresses: [email protected] (D. Cao), [email protected] (R. Guan).
http://dx.doi.org/10.1016/j.saa.2017.03.073 1386-1425/© 2017 Elsevier B.V. All rights reserved.
have long reaction time and molecular reaction potential barrier, the identification process needs a certain number of fluoride anions to break Si\\O bond. Graphene oxide (GO) possesses some unique excellent characteristrics [27,28], such as large surface area, high water dispersibility and catalytic action, which can absorb chemosensor molecules on its surface through π-π stacking and improves the hydrophilia of chemsensor as nanocarrier [29–32]. Our group has devoted to fluoride recognition for about ten years [33–35]. Here, a novel silicon-oxygen aurone derivative TBDPSA was synthesized and successfully assembled on the GO surface. As expected, the nanocomposite GO/ TBDPSA overcomes the disadvantage of long response time and low sensitivity of TBDPSA.
2. Experimental 2.1. Chemical and Instruments The synthetic routes to the title compound TBDPSA is shown in Scheme 1. 4′-Bromide-4-hydroxy-aurone (S-1) was synthesized according to ref. [35]. tert-Butyldiphenylsilyl chloride was obtained from Aladdin Reagent Inc. Nuclear magnetic resonance spectra were obtained on a Bruker Avance III 400 MHz spectrometer. Mass spectra were recorded on an Agilent Q-TOF6510 spectrometer. Scanning electron
38
Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 182 (2017) 37–41
ethanol solution with GO in HEPES buffer solution. Fluorescence spectra were obtained with the excitation wavelength being 389 nm. Both the concentration of pure TBDPSA and composite GO/TBDPSA used for spectral measurement is 10 μM. KF in HEPES buffer solution was used as fluoride anions source. Scheme 1. Synthetic routes to the title compound TBDPSA.
microscope (SEM) and energy-dispersive X-ray (EDS) analysis was recorded by FEI QUANTA FEG 250 scanning electron microscope.
2.5. Structure Determination Single crystal of compound TBDPSA was obtained by slow evaporation of the compound in dichloromethane-n-hexane mixed solution. The diffraction data were collected on a Bruker Smart Apex CCD area detector using a graphite mono-chromated Mo Kα radiation. Crystal data, data collections and structure refinements of TBDPSA are shown in Table S1.
2.2. The Synthesis of TBDPSA 3. Results and Discussion S-1 (0.10 g, 0.32 mmol), imidazole (0.027 g, 0.4 mmol) and 3 mL dichloromethane were poured in a 25 mL flask. After cooling to 0 °C, 0.11 mL (0.09 g, 0.33 mmol) tert-butyldiphenylsilyl chloride was dropped into the solution. And then the mixture was stirred at room temperature for 2 h. The crude product was evaporated under reduced pressure and then oil sample was obtained. The product was purified via column chromatography with ethyl acetate/petroleum ether (1:4) as eluent to give 0.11 g TBDPSA as a yellow solid (62% yield). 1H NMR (400 MHz, CDCl3), δ: 1.18 (s, 9H), 6.13 (d, J = 8.0 Hz, 1H), 6.76 (s, 1H), 6.78 (d, J = 8.0 Hz, 1H), 7.17 (t, J = 8.0 Hz, 1H), 7.38–747 (m, 6H), 7.57 (d, J = 8.4 Hz, 2H), 7.76 (d, J = 8.4 Hz, 2H), 7.77–7.79 (m, 4H). 13C NMR (100 MHz, CDCl3), δ: 19.88, 26.48, 104.96, 110.07, 113.13, 114.37, 123.94, 128.08, 130.35, 131.68, 132.20, 132.23, 132.69, 135.66, 137.78, 147.49, 154.87, 166.86, 182.50. MS for (M + Na)+: Calcd exact mass 577.0811; found 577.0741. 2.3. The Synthesis of Nanomaterial GO/TBDPSA GO was prepared with natural graphite as starting material using modified Hummer's method [36]. TBDPSA was added to the diluted stock solution of GO (0.5 mg/mL). After the mixture was stirred for 2 h, GO/TBDPSA and the dissociative TBDPSA were separated by dialysis way. Finally, the GO/TBDPSA product was dried under vacuum at 60 °C. The chemical composition and surface structure of GO and GO/TBDPSA were characterized by SEM and EDS spectroscopy. 2.4. Photophysical Properties' Measurements UV–vis absorption and fluorescence spectra with C = 10 μM at room temperature were obtained on a Shimadzu UV2550 spectrophotometer and Edinburgh FLS920 spectrometer, respectively. GO/TBDPSA solution used for spectral measurement was prepared by mixing TBDPSA in
3.1. Synthesis and Molecular Structure The compound was synthesized by the reaction between tertbutyldiphenylsilyl chloride and hydroxy in 4′-substituted 4-hydroxyaurone. TBDPSA was purified and fully characterized via 1H NMR, 13C NMR, mass spectrum and crystal structure. The molecular structure of TBDPSA is shown in Fig. 1. The molecular belongs to triclinic system and P1 space group. The molecular backbone exhibits perfect planarity with the dihedral angle between benzofuran and benzene group being 8.87°, which is similar with that of the hydroxyl aurone precursor [35]. 3.2. Response of the Compound to Fluoride Anions The compound TBDPSA was dissolved in aq. HEPES buffer-ethanol solution (1:1, v/v) and fluoride anions was progressively added in the solution. UV–vis absorption and fluorescence titration spectra are shown in Fig. 2. As shown in the Fig. 2a, the compound exhibits strong UV–vis absorption in blue light region with absorption peak at 305 nm (ε = 4.68 × 104 M−1 cm−1) and 389 nm (ε = 4.24 × 104 M−1 cm−1) in aq. HEPES buffer-ethanol solution (1:1, v/v). Upon the addition of F−, the original absorption peak at 389 nm gradually decreases with a little bathochromic shift. TBDPSA solution would reach saturation when 1000 equiv. F− was added. The fluorescence spectra of the compound also changes obviously during the titration with F−, which was shown in Fig. 2b. The fluorescence peak at 498 nm gradually decreases and is quenched finally. During the titration with KF, the change can be seen by naked eye under UV lamp. Green fluorescence is almost quenched. The bonding constant is calculated to be 3.9 × 104 M−1 [37]. The detect limits of the compound are 76 μM and 0.22 μM with absorption and fluorescence as detected signal, respectively. U.S. Environmental Protection Agency limit the fluoride anions in drinking water are
Fig. 1. Molecular structure of the title compound TBDPSA.
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didn't lead to obvious change in the absorption and fluorescence spectra of the compound. Other anions also didn't interfere with the recognition reaction between TBDPSA and F−. 3.3. The Composite GO/TBDPSA Response to Fluoride Anions GO and GO/TBDPSA were first investigated by SEM and EDS spectroscopy (Fig. S3 and S4). As shown in Fig. S3, GO and GO/TBDPSA exhibit good film structure. Comparing with the different EDS peaks between GO and GO/TBDPSA (Fig. S4), there are two more peaks represent Si and Br elements, which just come from chemosensor TBDPSA. As shown in Fig. S5, GO dose not exhibit obvious UV–vis absorption band from 300 nm to 600 nm. GO/TBDPSA exhibits two absorption bands at 309 nm and 389 nm, which is similar to that of TBDPSA. EDS and UV– vis absorption data prove the successful assembly of the GO/TBDPSA composite. TBDPSA exhibits low sensitivity and slow response rate for F− (about 1 h) with rate constant being 1.15 × 10−3 s−1 (Fig. S1). Based on superior catalytic and carriers function from Graphene Oxide (GO), GO was imported to TBDPSA aqueous solution to assistant recognizing F−. As shown in Fig. 4a, GO/TBDPSA solution is at the same concentration with previous titration (C = 10 μM) and only 220 equiv. F− can make the reaction reach saturation. The bonding constant is calculated to 1.2 × 105 M, which is three times that of TBDPSA. The import of GO immensely increases the recognition sensitivity of TBDPSA for fluoride anions. The fluorescence spectra of GO/TBDPSA (Fig. 4b) show the
Fig. 2. UV–vis absorption (a) and fluorescence (b, λex = 389 nm) spectral changes of TBDPSA upon titration with KF in aq.-HEPES buffer-ethanol (VHEPES:VEtOH = 1:1) with equilibrium time being 1 h. Insert: absorbance at 389 nm (a) and fluorescence intensity at 498 nm (b) as a function of concentration of KF. Photographs showing the fluorescence change upon the addition of KF in 365 nm UV-light.
0.7–1.2 ppm (37.1–63.6 μM) [38]. The detect limit [39] with absorption as detected signal does not meet the requirement. Although the detect limit with fluorescence as detected signal reaches the requirement from U.S. Environmental Protection Agency, but the response time is too long (about 1 h, Fig. S1). The selectivity and competitive experiments of TBDPSA for F− and other anions were investigated, which is shown in Fig. 3 and Fig. S2. Different from fluoride anions, other anions
Fig. 3. Fluorescence spectral changes of TBDPSA in aq.-HEPES buffer-ethanol (VHEPES:VEtOH = 1:1) upon the addition of various anions. Insert: Changes of fluorescence peak at 498 nm in the presence of various anions in response to F−.
Fig. 4. UV–vis absorption (a) and fluorescence (b, λex = 389 nm) spectral changes of GO/ TBDPSA upon titration with KF in aq.-HEPES buffer-ethanol (VHEPES:VEtOH = 1:1). Insert: absorbance at 389 nm (a) and fluorescence intensity at 492 nm (b) as a function of concentration of KF. Photographs showing the fluorescence change upon the addition of KF in 365 nm UV-light.
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Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 182 (2017) 37–41
Fig. 5. The reaction mechanism of GO/TBDPSA to detect F−.
same catalysis in the same situation. Response rate increases smartly and the response time is b5 s. The detect limits of GO/TBDPSA are 42 μM and 0.058 μM with absorption and fluorescence as detected signal, respectively, which satisfies the requirement from U.S. Environmental Protection Agency. So GO can improve the recognition speed for fluoride anions and the sensitivity. GO/TBDPSA composite also exhibits perfect selectivity for F− (Fig. S6).
and Universities Science and Technology Foundation of Shandong Province (J16LA08) and the Fund of Graduate Innovation Foundation of University of Jinan, GIFUJN (S1504). Appendix A. Supplementary Data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2017.03.073.
3.4. Reaction Mechanism References In situ 1H NMR spectra of TBDPSA before and after the addition of F− in CD3OD (Fig. S7) were measured to investigate the recognition mechanism. From Fig. S7, one can see that hydrogen proton signals of the compound at 6.78 ppm (Cα, s) and 6.90 (C5, d, J = 8.0 Hz) are upshifted to 6.50 ppm (Cα, s) and 6.26 (C5, d, J = 8.0 Hz), respectively. Other hydrogen proton signals do not exhibit obvious change in 1H NMR spectra. The possible recognition mechanism based on the in situ 1 H NMR spectra data is shown in Fig. 5. Recognition mechanism relies on the different binding energy between Si\\O bond and Si\\F bond. Si tended to separate with O and combined with F during the titration. As shown in the Fig. 5, GO/TBDPSA would occur permutation reaction and fluoride anions would replace aurone part and combine with TBDPSA. So that Si\\O bond was separated because of higher bond energy of Si\\F. Finally, the whole structure changed and caused red shift and fluorescence was quenched. 4. Conclusion In conclusion, a novel silicon-oxygen aurone derivative TBDPSA was synthesized and successfully assembled on the GO to form GO/TBDPSA composite material. Graphene oxide modified composite GO/TBDPSA exhibits higher sensitivity and fast response rate for fluoride anions compared with TBDPSA in aqueous solution because of the catalytic effect from GO. Generally speaking, GO is an excellent promoter for accelerate recognition. Acknowledgements This work was supported by the National Natural Science Foundation of China (21672130, 51373069, 21601065) and the Natural Science Foundation of Shandong Province (ZR2015BL011), Key Research and Development Plan of Shandong Province (2016GSF117004), Colleges
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